1. Field of the Invention
The invention is in the field of providing cable-ducts in the ground. More particularly, it relates to a method and a device for installing a cable-duct around a cable buried in the ground. It furthermore relates to a method for installing a cable-duct in the ground by making use of a cable buried in the ground.
2. Prior Art
At present, a transition is taking place in telecommunication networks from electrical signal transmission to optical signal transmission on an ever greater scale. In this transition, electrical cables, in this case copper cables, are being replaced by optical fibre cables. In contrast to copper cables, optical fibre cables are installed in the ground not directly, but in special cable-ducts, usually in the form of plastics tubes, inter alia, because of their greater vulnerability and greater weld-free lengths. If a copper cable has to be replaced by an optical fibre cable, the copper cable can be dug up and replaced by such a plastics tubular cable-duct into which the optical fibre cable is later introduced and installed. Since the digging of trenches in the ground is expensive and time-consuming, trench-free installation techniques are also used, depending on the circumstances.
Various trench-free installation techniques are known for installing such cable-ducts in the ground. Thus, for example, reference the TERRABOR.RTM. 2001 Directional Drilling System (Craelius, E 110388) is a technique for drilling with the aid of a drilling head mounted on the foremost end of a drilling rod which is formed by a number of coupled, hollow tubular drilling-rod sections. In this technique, the drilling head is pushed, optionally together with the drilling rod into the ground, while rotating, and at the same time a drilling liquid is forced through the hollow drilling rod and the drilling head. During the progress of the drilling, new drilling-rod sections are continually coupled on at the rear. During this operation, the drilling head is tracked from above the ground radiographically and is guided via the drilling rod. Once the drilling rod has reached a desired aboveground end point, a cable-duct to be installed is attached, in the end point, to the foremost end of the drilling rod. The drilling rod is then withdrawn from the drilled hole, and the cable-duct is pulled into the drilled hole, optionally preceded by a reamer. Such a guided drilling technique has the advantage that the existing infrastructure, such as waterways, motorways and railways, do not have to be disturbed. This known technique does, however, have a number of disadvantages. It is fairly expensive, inter alia because of the navigation system required for the drilling head. In urban areas, moreover, there is an increased risk of damage to existing water lines, gas lines, electricity lines and other lines, inter alia because of the limited accuracy of the navigation. Still further the drilling rods require a certain stiffness which appreciably limits the sharpness of bends in the drilling route. In addition, in the event of replacement, discarded copper cables remain behind in the ground, and this is undesirable, inter alia, for environmental considerations.
DE-A-3331291, GB-A-2103888 and GB-A-02085670 disclose a technique for replacing ground cables, in which a specific digging apparatus is used which advances around the ground cable in the axial direction under hydraulic drive and which can loosen and flush away the ground around the cable to be replaced, after which the cable can easily be pulled out of the ground. The apparatus is provided with special nozzles for a flushing liquid which is fed through a liquid line which the apparatus carries along parallel to the cable as it advances. Parallel thereto, the apparatus also carries along two feedlines for feeding an hydraulic liquid for the hydraulic drive. During the extraction, the replacement cable is pulled into the ground at the end of the cable to be replaced. This known technique can be used for installing cable-ducts if a cable-duct is pulled in place of a cable. Advantages of such a method of installing cable-ducts in the ground are that a navigation system can be dispensed with, that the risk of damage to other lines in the ground is minimal, and that the cable to be replaced does not remain unused in the ground as an interfering object and can also be recycled. A disadvantage is, however, that the lines carried along by the apparatus have to be pulled through the drilled hole over the entire length in an unprotected manner, and must be removed after loosening the cable in the ground prior to, or at the same time as, the withdrawal of the cable. Under these circumstances, there is an additional risk that the drilled hole formed around the loosened cable is affected, in particular if the drilled hole is rather tortuous.
DE-A-3331291 discloses a technique for laying ducts in the ground in a trench-free manner, in which a tube is slid into the ground over an existing cable. This technique uses a so-called ramboring apparatus which is pneumatically driven to advance coaxially over the cable, with the ground around the cable being pushed aside. While advancing, the apparatus pulls a tube connected to it, which slides along over the cable freed from ground. Through the tube, feedlines for compressed air and possibly a lubricant are also conveyed. The pneumatic drive occurs by means of piston-cylinders mounted in the apparatus which carry out an oscillating strike movement under compressed air drive, as a result of which the apparatus slides forward over the cable jerkily. With such a technique, a cable-duct can basically be installed into the ground without excavation, after having installed a tubular cable-duct around an old cable, by removing said cable thereout after which the cable-duct is ready for inserting a new cable. This technique also has the advantages that no navigation system is needed, that the risks of damage to other cables in the ground is minimal, and that the old cable can be removed in a simple manner. An additional advantage is that the feedlines carried along are situated in a protected manner in the tube drawn by the apparatus. However, the technique has a number of limitations. The pulling forces exerted by the ram-boring apparatus on the foremost end of the tube can soon be insufficient to be able to slide a tube, steadily increasing in length, along over the cable through the drilled hole in the ground, especially if said cable is quite tortuously situated in the ground. Moreover, the use of a pneumatically driven ramboring apparatus is not always suitable. The apparatus for example may not operate well, or may not operate at all, in soft ground, and vibrations can occur in hard ground due to the pneumatic strikes. Such vibrations can be harmful to possible buildings in the surrounding area. Furthermore, carrying along the feedlines requires a relatively large inner diameter for the tube with respect to that of the cable. Finally, the means for scraping dirt, which means are provided at the front of the apparatus, can impede the advance of the tube.